Mobile communication systems have evolved to the third generation (3G). The standards of Long Term Evolution (LTE) systems are now being developed in order to keep long-term competitiveness of the systems. Carrier aggregation is introduced into LTE Release 10 (R10) to further improve spectrum efficiency and user throughput of the system. Carrier aggregation means that a UE can use multiple Component Carriers (CCs) simultaneously for uplink and downlink communication.
Carrier aggregation is categorized into symmetric carrier aggregation and asymmetric carrier aggregation depending on whether the number of carriers used in the uplink direction is the same as the number of carriers used in the downlink direction. Regardless of the aggregation mode, the UE may communicate with multiple CCs in uplink and downlink directions. Because the cell on each CC carries its own synchronization channel and broadcast channel, the UE using multiple CCs is equivalent to communicating with multiple cells.
Cells applied in a centralized geographic area are called “multiple centralized cells”. A set of multiple centralized cells is a centralized cell set. FIG. 1 shows carrier aggregation of centralized cells. As shown in FIG. 1, four cells centralized in the geographic location (namely, under the same coverage) send signals through four CCs of different frequencies respectively. The UE communicates with the four cells by communicating with the four CCs.
To improve experience of edge users, a Coordinated Multi-Point transmission (CoMP) technology is introduced into the LTE R10. FIG. 2 shows principles of CoMP. As shown in FIG. 2, a UE may receive and send data through multiple cells. The cells may be distributed under the same evolved NodeB (eNB), or under different eNBs. To achieve the best uplink and downlink communication performance, the UE communicates with multiple different cells in the uplink and downlink directions. The multiple cells which are separated geographically are called “multiple distributed cells”. The CoMP is a structure of multiple distributed cells. A set of multiple distributed cells is a distributed cell set.
After the centralized multi-cell structure represented by carrier aggregation and the distributed cell structure represented by CoMP are introduced, a concept of anchor cells is introduced in order to manage connections and share resources more efficiently. An anchor cell is a cell that sends Packet Data Control Channel (PDCCH) messages.
After CC and CoMP are introduced, the UE is served by multiple cells in much more scenarios. In the prior art, when a UE communicates with multiple cells, each cell manages mobility for the UE separately, which is rather complicated in maintaining the mobility performance, especially when the UE communicates with different cell sets in uplink and downlink directions.